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The advanced interface to the validator at [1] does not allow to select XHTML 1.1 or XHTML Basic 1.0. It should support at least the former.
Fixed. Added XHTML 1.1 and XHTML Basic 1.0 to the dropdown.
Not sure why this had been reopened, but after a quick check, I can confirm that the two document types are indeed present. RE-closing.